During the design and manufacture of components of systems that are to be used within particular industries, important concerns include surface smoothness, hardness, cost and environmental effects of fabrication techniques. As one example, the semiconductor fabrication industry utilizes ultra high purity gas delivery systems in which these concerns are considered in the selection of techniques for manufacturing the system components. Such system components include mass flow controllers, valves, pressure regulators, purifiers, filters and tubing. In the semiconductor industry, corrosion resistance in a gas delivery system is critical to achieving and maintaining a contaminant-free environment. Smoothness plays an important role in controlling turbulence and minimizing moisture-inducing conditions. There are detailed, strict specifications on the cleanliness, smoothness and hardness of materials used in manufacturing components of gas delivery systems, as specified by organizations such as Semiconductor Equipment and Materials International (SEMI). Other industries in which the concerns are important factors include those of the medical and aerospace fields.
Many gas delivery components must meet a Vickers hardness of 300, which is a standard intended to ensure that metal surfaces will not generate scratches upon meeting with each other. Scratching is an issue, since it leads to the generation of particulates. Materials that are selected because they satisfy other requirements may not meet this standard and, therefore, are subjected to elaborate post-manufacturing techniques designed to increase hardness. For example, 316L stainless steel, which is an austenitic alloy made of iron, chromium, nickel and other trace materials, does not intrinsically meet the 300 Vickers hardness requirement until the component is burnished. Unfortunately, burnishing is time-intensive and significantly affects the yield of a manufacturing batch of the components.
One approach to collectively addressing the concerns is to select a base material for certain properties and then coat the base material following shaping of the component. Typically, the coating material is either a metal or a metal-containing compound (such as a ceramic). Coating techniques include chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma spray, electroplating, and sol-gel. For CVD, a gaseous species that includes the coating material may be introduced into a vacuum chamber, so that when the gas is decomposed in the chamber, the coating material is deposited upon the component (i.e., the “workpiece”). In comparison, PVD is a thin film deposition process in the gas phase, wherein the coating material is physically transferred in the vacuum without any chemical reactions. That is, there is no change in chemical composition of the coating material. PVD may be performed by evaporating a target of coating material in vacuum using heating, an ion beam, cathodic arc, or an electron beam. The cathodic arc process may employ a magnetic field to confine an arc having a high current density to the area of the target. By using the magnetic field to steer the arc, a greater portion of the target may be used before replacement is required.
The PVD process offers improved safety and environment conditions, as compared to the typical CVD process, when depositing a metal or metal-containing film, such as titanium, titanium nitride, chromium, and chromium nitride. PVD typically operates in the range of 1 mTorr to 10 mTorr. This is accomplished by using magnetic confinement of the plasma electrons near the cathode, the so called “sputter magnetron,” to enhance the plasma at low pressures. Non-magnetron forms of sputtering can be operated at higher pressures, for example diode sputtering, which can operate up to 1 Torr. In comparison, CVD pressures may be in the range of 50 mTorr to atmospheric, with low pressure CVD (LPCVD) typically being between 100 mTorr and 1 Torr. However, other process reasons may dictate the use of LPCVD. For example, the LPCVD approach may be selected over the PVD process for purposes of reducing the mean free pass and the directionality of gas flows. This may be important for applications in which the workpiece to be coated is one that has a complex geometry.
Coating a workpiece can be particularly problematic when the workpiece includes internal passages, such as those found in valves, pressure regulators, and tubes. As the complexity of the geometry of the workpiece increases, there is a decrease in the ability to select a coating process on the basis of maximizing environmental and human safety. U.S. Pat. No. 5,026,466 to Wesemeyer et al. describes a solution which may be used in a limited number of applications for coating internal passageways. In Wesemeyer et al., the cathode (i.e., the target formed of the coating material) is positioned within the cavity of the workpiece. For example, when the workpiece is a tube, the cathode is positioned within the internal passageway through the tube. In operation, the material from the cathode surface is evaporated and is deposited on the internal surface of the workpiece. Optionally, the workpiece is connected as the anode. That is, a negative voltage may be applied to the workpiece so as to provide the condition for inducing evaporation of material from the cathode located within the workpiece.
While the Wesemeyer et al. patent and patents to Gorokhovsky (U.S. Pat. Nos. 5,435,900 and 6,663,755) describe coating apparatus that perform well in various applications, performance factors such as coating uniformity may be significantly affected when the workpiece is geometrically complex.